Concrete Masonry Units (CMU Cinder Blocks / ASTM C90)
Direct Answer
Concrete Masonry Units (CMU), commonly called cinder blocks or concrete blocks, are standardized hollow load-bearing structural blocks governed by ASTM C90. A standard nominal 8×8×16-inch block is manufactured to actual dimensions of 7-5/8 × 7-5/8 × 15-5/8 inches (194 × 194 × 397 mm), designed to incorporate standard 3/8-inch (9.5 mm) mortar joints into a precise 8-inch modular grid.
Source: ASTM C90-22 (Standard Specification for Loadbearing Concrete Masonry Units)
Specs
| Governing Standard | ASTM C90 / NCMA TEK / CSA A165.1 |
| Standard 8" Block (Nominal) | 8 × 8 × 16 inches (203 × 203 × 406 mm) |
| Standard 8" Block (Actual) | 7-5/8 × 7-5/8 × 15-5/8 inches (194 × 194 × 397 mm) |
| Standard Mortar Joint Thickness | 3/8 inch (9.5 mm) bed and head joints |
| 4" Stretcher Block (Actual) | 3-5/8 × 7-5/8 × 15-5/8 inches (92 × 194 × 397 mm) |
| 6" Stretcher Block (Actual) | 5-5/8 × 7-5/8 × 15-5/8 inches (143 × 194 × 397 mm) |
| 12" Stretcher Block (Actual) | 11-5/8 × 7-5/8 × 15-5/8 inches (295 × 194 × 397 mm) |
| Compressive Strength Minimum | 2,000 PSI (13.8 MPa) net area average compressive strength |
The 3/8-Inch Mortar Joint and 8-Inch Modular Grid
In architectural drafting and masonry construction, all dimensions are planned on an $8\text{-inch (203 mm)}$ modular grid. Because every block is manufactured exactly $3/8\text{ inch}$ ($9.5\text{ mm}$) smaller than its nominal name in all three axes ($7\text{-}5/8\text{" actual}$), adding the standard $3/8\text{-inch}$ mortar joint results in a finished block-plus-joint dimension of exactly $8.0\text{ inches}$ high and $16.0\text{ inches}$ long ($7\text{-}5/8 + 3/8 = 8\text{"}$). This eliminates block cutting when framing window and door rough openings.
Hollow Core Geometry, Rebar, and Grouting
Standard two-core CMU blocks feature tapered interior cell walls with thicker face shells at the top to provide a larger mortar bed. Vertical steel reinforcing bars (rebar, typically #4 or #5) are placed inside aligned cores and filled with fluid pea-gravel concrete (grout, $\ge 2,000\text{ PSI}$) to create composite reinforced masonry walls capable of resisting extreme hurricane wind and seismic shear forces.